1. Field of Invention
The present invention relates to a method and an apparatus for dispensing paint powders for powder coatings, and more particularly, the present invention relates to a corona charge spray gun for dispensing paint powders for powder coatings.
2. Description of the Related Art
Electrostatic powder coating is a method of surface finishing for metals or other materials in which a paint layer is applied in a dry powder form without the use of solvents. The powder, usually having powder particles with a mean size of about 30-60 microns and composed of a resin, pigments, flow agents and curing agents etc., is fluidized in a hopper and pneumatically transported to a spray gun through a plastic or rubber hose. The powder is then sprayed out the exit passageway of the gun whereupon it is positively or negatively charged and is attracted to a grounded work-piece, whereupon if forms a uniform powder layer. The work-piece covered with powder paint is then transferred to an oven where the powder layer melts and certain chemical reactions occur to form a smooth film of paint.
The spray gun can be a corona charge gun which is most widely used in the coating industry, or a tribo charge gun which occupies only a small fraction of the total market share. Conventional corona charge spray guns have a configuration similar to that shown in FIG. 1a which includes a powder-air mixture conduit, a high-voltage needle-like electrode located at the gun tip and a powder diffuser. Another typical configuration includes a powder-air mixture conduit in the side of a central gun housing midway of the housing for injecting the powder-air mixture into a chamber, as shown in FIG. 1b. A pointed needle or charge pin is connected to a high voltage generator which typically imparts a negative potential to the electrode. An electric field is established between the needle electrode and the grounded work-piece, with an intensified electric field located at the needle tip due to its small radius of curvature. When a combination of needle geometry and potential is sufficient to create a local electric field strength of 3 MV/m or higher, electrical breakdown of the air, or corona discharge, occurs in a region around the electrode tip. As a result of the local discharge, the air will be ionized, producing negatively charged ions. Powder particles carried by compressed air are transported along the conduit and pass through a discharge region, picking up negative ions on their way to the work-piece.
Corona charge guns, although are widely used in the coating industry, suffer from problems such as Faraday cage effect and back ionization. It would be desirable to provide an apparatus for dispensing powders while avoiding or minimizing the Faraday cage effect and back ionization. When workpieces with a convex geometry have to be coated by corona guns, the presence of an electric field between the gun tip and the work-piece generates a very serious problem, namely poor powder coverage in recessed areas coupled with excessive building up of powder in areas of boundaries or edges. This is a direct result of classical electrostatics, namely, less or no electric field lines can exist or penetrate areas which are surrounded by a grounded metal boundary. If air velocity is low, particles will follow a field line pattern that does not penetrate into an inside of a recessed or concave area of a work-piece. As a general rule, electrostatic forces will deposit material into an opening to a depth equal to or less than a smallest dimension of the opening. This is known as the Faraday cage effect. To a certain extent, a higher air velocity will help by “pushing” a powder into recessed or concave areas but this does not compensate for poor uniformity of coverage.
To eliminate or significantly reduce problems caused by the Faraday cage effect, alternative configurations of corona charge guns have been proposed and/or patented. Included in these are internal charging guns which charge the powder internally in the gun barrel before the powder is ejected from a gun outlet. Since there are no electrical lines built up between the gun nozzle and the grounded work-piece, the Faraday cage effect is eliminated. It is noted that given that the charged powder coming out of the gun tip also generates electrical potential, there may exist a weak electrical field between the gun tip and the work-piece, but such an effect is negligible. As referred to by Moyle, B. D. and Hughes, J. F. (Electrostatics, 16, 277, 1985), an internal charging gun comprises a duct in which a corona discharge needle electrode is located, an grounded ring electrode surrounding the tip of the corona needle or located downstream of the needle, as indicated in FIG. 1c. All powder emanating from the gun nozzle will pass through the corona discharge region surrounding the needle tip and charging is imparted to the powder in this region. Free ions not captured by the powder will be attracted to the surface of the grounded counter electrodes so that few of them are ejected from the nozzle. The result is a high specific charge with a fairly small voltage on a corona electrode, an electrical line free space between the gun nozzle and work-piece, and a large reduction of free ion emission towards the work-piece.
As stated by Moyle, B. D. and Hughes, J. F. (Electrostatics, 16, 277, 1985), although remarkably good in terms of high-quality coatings, long-term tests with the prior art internal charging corona guns have shown deterioration in performance after long uninterrupted runs. This is considered to be associated with a growth of partially cured powder and back-ionization on a ground counter electrode inside the gun. The inventors have conducted tests showing that with this configuration of electrodes, the ground counter electrode is coated by powder and becomes back-ionized within a few seconds to a few minutes. Powder deposition results in a significant deterioration of the charging performance leading to a degradation and failure of the gun. Thus, frequent cleaning of the gun must be performed with utmost care. This is a time consuming operation which normally requires shutdown of a production line.
Considerable efforts have been made to improve long-term efficiency of internal charging guns. Two different approaches involving a piezo-electric ceramic ring electrode which undergoes an oscillatory deformation and a curtain electrode with a double helix configuration have been considered as a solution to the problem (Moyle, B. D. and Hughes, J. F., Electrostatics, 16, 277, 1985; Masuda, S. IEEE/IAS Conference Proceedings 35D, 1977, P. 887). However, as mentioned by Misev, T. A. (Powder Coatings Chemistry and Technology, John Wiley & Sons, 1990), despite the promising results, neither attempt resulted in a commercial gun system. Another alternative is to employ a porous metal ring electrode which is cleaned by air purge (Misev, T. A., Powder Coatings Chemistry and Technology, John Wiley & Sons, 1990) to remove powder coated on a ring surface. Again, this design found no wide application as the ring surface can not be thoroughly cleaned because of the nature of the porous surface.
A recent improvement to the internal charging gun has been made by Muhlhausen, B. G. and Heidelberg, H-G. N. etc., ABB Research Ltd., Zurich, Switzerland and is disclosed in U.S. Pat. No. 6,254,684B1 (continuation of PCT/EP96/05462, or WO98/245555). This design includes a chamber, several high-voltage electrodes annularly distributed in a region upstream of the outlet and a tubular ground electrode extending along the cylindrical axis of the chamber at the back of the gun housing. The tubular ground electrode has an end directed towards an interior of the chamber and is covered by an insulating material with a small hole through which the tubular ground electrode is exposed to the high-voltage electrodes. The purpose of this configuration is to prevent the charged powder from depositing on the ground counter electrode because the ground counter electrode is located upstream of the high-voltage electrodes and is also continuously flushed by clean air. However, this design suffers from two drawbacks. First, the area of the ground electrode exposed to the high-voltage electrodes is very small which can result in dangerous sparking during operation. Secondly, due to the small area of the exposed ground electrode, the intensity of electrical field at the high-voltage electrode may not reach sufficiently high strengths to generate enough free ions that charge the powder effectively.
German Patent No. 27 22 100 B1 also describes a spray gun having a blunt ground electrode in a section of the gun barrel having an enlarged diameter located upstream of the charging pin and centrally located in the flow passageway. The purpose of having the ground electrode with a blunt shape in the enlarged cross-sectional area is to cause powder flow to slow to allow time for powder charging. However, the inventors have conducted tests on such configurations which have shown that this structure results in a highly irregular passageway for the powder and causes a surface of the blunt ground electrode, especially the side facing the high voltage electrode, to be coated almost immediately, leading to performance failure of the gun.
It would be very advantageous to provide a powder spraying apparatus which overcomes the aforementioned disadvantages of the above designs that provides long-term efficient charging performance and with a relatively simple configuration.